Designing a system or optics for a spot lighting effect is in general challenging, especially for light sources which are not completely uniform in their light output over angle or position. There are many light sources available, for example LEDs can be grouped into high power LEDs, medium power LEDs, low power LEDs, clustered LEDs and chip on boards.
High power LEDs can be dimensionally small, for example with an area of around 1 mm2, and the phosphor of the LED can be well matched with the area of the die. This leads to a relatively uniform colored light distribution with respect to the emission angle.
Medium power and low power LEDs are often somewhat larger, for example with an area of 2-6 mm2. This use of medium or low power LEDs often leads to color distribution effects, because these LEDs have the disadvantage that they emit light in all directions and that their color is not uniform over the entire LED.
Beam shaping is found in most lighting applications. Beam shaping optical elements for example include reflectors and collimators, and these are used in most luminaires.
The problem of color distribution becomes severe when using medium power light emitting diodes (LEDs) for generating white light or Red-Green-Blue (RGB) LEDs for generating color variable light. Even when such light sources are combined with optics having collimation characteristics, color distribution effects with respect to emission angle may result. This is a major issue and often obstructs the application of medium power LEDs.
When using LEDs in a spot light application, a collimated beam is desired and the color should be uniform over the entire spot.
A solution to make a collimated and uniform color beam from a divergent source with color non-uniformity is by first collimating the light with a collimator, and then mixing the color by means of a Koehler lens design.
A Koehler design consists of a double lens array, with one array on each side of a substrate, the two arrays together forming an optical plate. Both lens arrays are positioned in each other's focal plane and are aligned with respect to each other. The function of each Koehler lens pair is to re-distribute the light entering the first side of the substrate to remove angular information of the original beam spot.
Even with a collimator and optical plate, the output beam shape is difficult to control to have a desired intensity characteristic.